Braking and driving forces (or an inertial force) and/or other external forces, acting on a vehicle body at the acceleration and deceleration of a vehicle to cause pitching-bouncing vibration, etc. during the running of the vehicle, are reflected in a torque acting between a wheel (particularly a driving wheel during driving) and a road surface (referred to as “wheel torque” in this specification.). Then, in the field of the vibration damping control of a vehicle, it is proposed to suppress the vibration of a vehicle body during the running of a vehicle by adjusting a wheel torque through a control of a drive output of an engine or other driving device of the vehicle (For example, see Japanese Patent Laid-Open Publications 2004-168148 and 2006-69472). In such a vibration damping control through a drive output control, pitching-bouncing vibration, which arises in a vehicle body when an acceleration or deceleration demand of a vehicle is made or when an external force (a disturbance) has acted on a vehicle body to vary a wheel torque, is predicted by using a motion model constructed based upon an assumption of a dynamic model for the so-called sprung mass vibration or sprung mass and unsprung mass vibration of a vehicle body, and then, the drive output of a driving device of the vehicle is adjusted so as to suppress the predicted vibration. In the case of such a type of vibration damping control, the generation of vibrational energy is suppressed by the adjustment of the source of the vibration-generating force, rather than by absorbing the generated vibrational energy like in the vibration damping control by suspensions, and therefore, the relatively prompt vibration damping effect and good energy efficiency are advantageously obtained. Further, in the vibration damping control through a drive output control, the object to be controlled is focused into the drive output (driving torque) of a driving device, and thus, the adjustment in the control is relatively easy.
In such a vibration damping control device (or a driving force control device) performing vibration damping control through a drive output control as described above, the wheel torque actually generated in a driving wheel is to be used as an input parameter of a feedback for the suppression of a disturbance in the control. Generally, however, a sensor which can directly detect a wheel torque value during the running of a vehicle, e.g., a wheel torque sensor, a wheel's six force component meter, etc., is not installed on normal vehicles, except a vehicle for testing (see JP 2005-69897), due to problems in the designs or costs of vehicles. Thus, in a vibration damping control device as described above, an estimated wheel torque value, estimated based on a wheel speed, a rotational speed of an output shaft of a driving device of a vehicle and/or other easily detectable parameters, is employed for the wheel torque value to be fed back as a disturbance input. Such a wheel torque estimated value, obtained based on a wheel speed, etc., is monitored in various controls such as TRC (Traction Control), ABS, VSC (Vehicle Stability Control), VDIM (Vehicle Dynamics Integrated Management System) for the purposes of monitoring a road surface reaction force on a wheel or testing a running performance of a vehicle (For example, see JP H11-37872).